Generally speaking, a power supply employs a AC/DC converter receiving an inputting commercial AC power and converting the inputting commercial AC power into a DC power having a relatively high voltage level, and employs a DC/DC converter converting the DC power having the relatively high voltage level to a DC power having a relatively low voltage level. The DC power having a relatively low voltage level is employed to operate an electronic device, e.g., using as the power supplies of a desktop and a notebook computers.
Nowadays, the power supplies can be divided into two categories: the linear power supplies and the switching power supplies. Currently, the various switching power supplies are the main streams of the market.
In general, a switching power supply includes an input stage circuit, a PFC stage circuit, a power stage circuit and a feedback stage circuit. In which, the core of the switching power supply is the power stage circuit.
Please refer to FIG. 1, it is a schematic diagram showing the power stage circuit of a conventional switching power supply. In which, the power supply 10 is a forward switching power supply.
In FIG. 1, the switching power supply 10 includes an auxiliary capacitor Ca, a main switch Q1, an auxiliary switch Q2, a transformer T, a driver transformer T1, a zero-voltage switching (ZVS) detector IC 11, a first rectifying switch S1, a second rectifying switch S2, a filter inductor L and a filter capacitor Cb.
The operational principles of the switching power supply 10 of FIG. 1 are described as follows. The input voltage from the previous stage (the PFC stage circuit) is provided to the primary side of the transformer T firstly. The input voltage is rectifying into a DC voltage through the first rectifying switch S1 after the secondary side of the transformer sensing the inputting electrical energy secondly. After the filter inductor L filters the ripples of the DC voltage, the DC voltage is outputted thirdly. The polarities of the voltages on the windings of the transformer are reversed when the main switch Q1 is turned off and the auxiliary switch Q2 is turned on such that the first rectifying switch S1 is turned off, the second rectifying switch S2 is turned on, and the electrical energies stored in the filter inductor L and the filter capacitor Cb are provided to the output terminal Vo through the second rectifying switch S2 fourthly.
Referring to the conventional switching power supply 10 as shown in FIG. 1, the ZVS detector IC 11 is employed to detect the main switch Q1 and cope with the driving of the driver transformer T1 to switch the auxiliary switch Q2 to achieve the alternated turn-on and turn-off of the main switch Q1 and the auxiliary switch Q2 so as to accomplish ZVS of the main switch Q1.
Please refer to FIG. 1, the input voltage V1 is around 400 V high voltage in general. The design of the ZVS detector IC 11, which fulfills the purpose of driving the driver transformer T1 under such a high voltage circumstances, is relatively quite complex.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the applicant finally conceived the switching power supply and ZVS method thereof.